Adjust the Package::Version class to handle an epoch that is set in the
version string and as recipe field.

An epoch in the version string has precedence over the recipe field for
backward compatibility.

If the epoch is set in both, the version string and the epoch field, we
throw an exception because we cannot select on value automatically.

Refs #210

Setting the field to a format string value changes the filename of the
created package. If unset, FPM chooses its default format.

See the following code section for available format strings:
https://github.com/jordansissel/fpm/blob/c40f6818f88a6dd3df7bc19b6007b03a716949f8/lib/fpm/package.rb#L355-L365

Fixes #210

- Add "before_package_file_create"
- Add "after_package_file_create"
- Deprecate "before_package_create"
- Deprecate "after_package_create"

The new hooks take the package filename as first parameter.

With the introduction of the package_name_format recipe field, the
package filename cannot be computed with the FPM::Package object alone
anymore. We now pass in the filename directly so consumers don't have to
compute the filename anymore.

We could have passed in the recipe object or the package name format so
consumers could compute the filename based on that, but that would
require deeper knowledge of the FPM::Package object.